Certain research and manufacturing processes require the use of a process chamber with high vacuum. For example, in semiconductor wafer processing, vacuum is used during the thin-film deposition and etching operations, primarily to reduce contamination. In such processes, pumps capable of producing a “high vacuum” of 10−6 torr or lower are useful to assure adequate pumping speed at process pressure, and to allow for a low base pressure for cleanup between steps.
Several currently-available vacuum pump configurations are capable of maintaining a high vacuum. One design, the turbo-molecular vacuum pump, is frequently used in both manufacturing processes and in research instrumentation. A conventional stage arrangement of a turbo-molecular vacuum pump includes a stack of alternate rotors and stators. Each stage effectively comprises a solid disc with a plurality of blades depending (nominally) radially inwardly or outwardly therefrom. The blades are evenly spaced around the circumference of the disc and angled “about” radial lines out of the plane of the disc in the direction of rotation of the rotor stage.
The rotor and stator blades have positive and negative gradients respectively when viewed from the side in a radial line from the disc. That arrangement has the effect, in molecular flow conditions, of causing a movement of molecules through the pump.
The turbo-molecular vacuum pump is inefficient or inoperable outside the molecular flow realm. For that reason, a commercially available vacuum pump may contain, in addition to several turbo-molecular stages, one or more molecular drag stages and one or more regenerative stages placed between the turbo-molecular stages and the pump outlet.
Referring to FIG. 1, there is illustrated a known compound vacuum pump 100 comprising a turbo-molecular section 50, a regenerative section 1 and a molecular drag section 2. Attached to a rotor 9 is a cylindrical rotor body 52 of the turbo-molecular section 50, placed at an inlet 31 of the pump. Extending radially outwardly from the rotor body 52 are rotor vanes 54 which collectively define three spaced arrays of vanes, each array having approximately 20 vanes.
The turbo-molecular section 50 also includes a stator 56 which is formed integrally with a body 22. Extending radially inwardly from the stator 56 are stator vanes 58 defining three spaced arrays of vanes, each array including about 20 vanes. The vanes 54 of the rotor interleaf with the vanes 58 of the stator and are angled relative to each other as is known in the turbo-molecular vacuum pump art.
The rotor 9 also comprises the rotor portion of the regenerative section 1. That portion includes a series of concentric rows of airfoils such as airfoil 10 that induce motion to the gas contained in channels 11. The channels 11 are stationary and are attached to the body 22 and stator 56. The regenerative portion of the pump is arranged to exhaust through the pump outlet 32.
The molecular drag section of the exemplary prior art pump illustrated in FIG. 1 is a Holweck section that includes cylindrical rotor sections 26, 27 and cylindrical stator sections 23, 24. The rotor sections 26, 27 are attached to the pump rotor 9, while the stator sections 23, 24 are attached to the pump stator. Helical grooves in the rotors and stators interact at close running clearances, transferring air molecules from the turbo-molecular section to the regenerative section.
A high-speed drive (not shown) is connected to a shaft 60 to drive the rotating components of the pump. The shaft, rotors and drive are supported by high speed, low friction bearings (not shown) such as dry ceramic race ball bearings or magnetic levitation bearings.
Operation of the turbo-molecular section of the above-described system requires high relative velocity of the turbo-molecular stators and rotors. On the other hand, the angular velocity of the rotor is limited by stresses placed on the components by centripetal acceleration, and also by maximum operating speeds of the other pump sections. Special fasteners and larger component cross sections are sometimes used to reduce stress and to permit a higher rotational speed.
The pumping speed requirements of the turbo-molecular section are often met by increasing the overall diameter of the pump, imparting a greater tangential velocity to the outer portions of the rotor vanes for a given rotational speed. That solution, however, increases the overall package size.
To start the above-described pump, the enclosed volume of the turbo-molecular section must first be partially evacuated. That is done to permit rotation of the opposing vanes in that section at the required high speeds. An additional downstream “roughing” pump is sometimes used for that purpose. A valved bypass line may be provided to permit the system to be pumped down without the turbo offering a restriction.
There is therefore presently a need to provide an improved compound or hybrid vacuum pump including a turbo-molecular pump section. Particularly, the improved pump should provide a solution to the conflicting speed requirements, and should reduce the need to evacuate the pump at start-up using a roughing pump. To the inventor's knowledge, there is currently no such technique available.